The frq locus in Neurospora crassa: a key element in circadian clock organization.

Four new circadian clock mutants of Neurospora crassa have been isolated that alter the period length of the circadian conidiation rhythm. Three of these are at the frq locus on linkage group VIIR, where four other clock mutants are located. In contrast to wild type, which has a period length of 21.6 hr, frq-6 has a period length of 19 hr, while frq-7 and frq-8 have period lengths of 29 hr and represent the largest effects of any single gene mutants on circadian periodicity. Thus, seven mutants have now been isolated that map to the frq locus, with period lengths ranging from 16.5 to 29 hr, and each mutant alters clock periodicity by an integral multiple of 2.5 hr. In addition, all frq mutants show incomplete dominance in heterokaryons. The large percentage of clock mutants that map to this locus, coupled with their unique properties, suggests that the frq locus plays an important role in clock organization.--The fourth mutant, designated chrono (chr), has a period length of 23.5 hr, shows incomplete dominance and is unlinked to either of the previously identified clock loci, frq or prd (formerly called frq-5). Double mutants between various combinations of clock mutants show additive effects and indicate no significant gene interaction among mutants at these three loci.